4.1.2. Effect of Carburizing Temperature on Surface Carbon Flux

Based on the effect of carburizing pressure in the previous section, the mass increment, average carbon flux, and segmented average carbon flux variations at different carburizing temperatures were investigated.

Figure 10 shows the surface mass increment, average carbon flux, and segmented average carbon flux with carburizing time for 12Cr2Ni4A steel under a carburizing pressure of 300 Pa at carburizing temperatures of 920, 950, and 980 ◦C.

**Figure 10.** Effect of carburizing temperature on (**a**) mass increment, (**b**) average carbon flux, and (**c**) segmented average carbon flux of 12Cr2Ni4 steel under 300 Pa.

During the boost stage for all carburizing temperatures, the mass linearly increased relatively quickly for 12Cr2Ni4A steel from 0 to 60 s. From 60 to 120 s, the mass increment gradually decreased and approached zero after 120 s. The average carbon flux value decreased by 50%–60%, and the segmented average carbon flux value decreased by ~80%. On increasing the carburizing temperature from 920 ◦C to 980 ◦C, the segmented average carbon flux value of the surface also increased from 1.18 <sup>×</sup> 10−<sup>5</sup> to 1.72 <sup>×</sup> 10−<sup>5</sup> at 30 s and from 3.38 <sup>×</sup> 10−<sup>6</sup> to 7.48 <sup>×</sup> 10−<sup>6</sup> at 60 s. At 90 s, the segmented average carbon flux values at 920 and 950 ◦C were 2.53 <sup>×</sup> <sup>10</sup>−<sup>6</sup> and 2.48 <sup>×</sup> <sup>10</sup><sup>−</sup>6, respectively. The carbon flux corresponding to 980 ◦C was 3.94 <sup>×</sup> 10<sup>−</sup>6. From 120 to 150 s, the segmented average carbon flux remained nearly identical at all three pressures, exhibiting relatively small flux values.

Figure 11 shows the surface mass increment, average carbon flux, and segmented average carbon flux with carburizing time for 16Cr3NiWMoVNbE steel under a carburizing pressure of 300 Pa at the carburizing temperatures of 920, 950, and 980 ◦C.

**Figure 11.** Effect of carburizing temperature on (**a**) mass increment, (**b**) average carbon flux, and (**c**) segmented average carbon flux of 16Cr3NiWMoVNbE steel under 300 Pa.

The mass linearly increased relatively quickly for 16Cr3NiWMoVNbE steel from 0 to 30 s. From 60 to 90 s, the mass increment gradually decreased and approached zero after 90 s. On increasing the carburizing temperature from 920 to 980 ◦C, the segmented average carbon flux value of the surface also increased from 1.49 <sup>×</sup> 10−<sup>5</sup> to 2.26 <sup>×</sup> 10−<sup>5</sup> at 30 s and from 3.86 <sup>×</sup> 10−<sup>6</sup> to 5.78 <sup>×</sup> 10−<sup>6</sup> at 60 s, respectively. At 90 s, the segmented average carbon flux values at 920 and 950 ◦C were 1.6 <sup>×</sup> <sup>10</sup>−<sup>6</sup> and 1.8 <sup>×</sup> <sup>10</sup><sup>−</sup>6, respectively, i.e., they were practically identical. The carbon flux at 980 ◦C had a minimum of 1.38 <sup>×</sup> 10−6. From 120 to 150 s, the segmented average carbon flux exhibited relatively small flux values under all temperatures.

Figure 12 shows the surface mass increment, average carbon flux, and segmented average carbon flux with carburizing time for 18Cr2Ni4WA steel under a carburizing pressure of 300 Pa at different carburizing temperatures. For 16Cr3NiWMoVNbE steel, the mass linearly increased relatively quickly from 0 to 60 s. From 60 to 150 s, the mass increment gradually decreased and approached zero after 150 s. On increasing the carburizing temperature from 920 to 980 ◦C, the segmented average carbon flux value of the surface also increased: from 1.19 <sup>×</sup> <sup>10</sup>−<sup>5</sup> to 1.98 <sup>×</sup> <sup>10</sup>−<sup>5</sup> at 30 s, and from 3.88 <sup>×</sup> <sup>10</sup>−<sup>6</sup> to 4.6 <sup>×</sup> <sup>10</sup>−<sup>6</sup> at 60 s. From 90 to 120 s, the segmented average carbon flux value at all three temperatures was relatively small and consistent.

**Figure 12.** Effect of carburizing temperature on (**a**) mass increment, (**b**) average carbon flux, and (**c**) segmented average carbon flux of 18Cr2Ni4WA steel under 300 Pa.

As can be observed in Figures 10–12, the average carbon flux and segmented carbon flux show the same trend for all three materials at the same holding temperature. Under the same holding pressure, with an increase of carburization temperature, the average carbon flux, and the segmented average carbon flux both gradually increased.

As carburizing proceeded, carbon atoms gradually accumulated on the steel surface until saturation was attained, and the mass growth became gradually slower until equilibrium was reached. The main driving force for carburizing is the carbon concentration gradient at the gas–solid interface, where the carbon concentration in the atmosphere is in a state of supersaturation. During carburization, the carbon concentration on the steel surface gradually increased, causing the concentration gradient to decrease slowly, thereby reducing the driving force as well as the effective carbon flux value.

As the carburizing temperature increased, the maximum solid solubility of carbon in austenite gradually increased, along with the carbon concentration gradient on the surface of the carburized layer. The steel surface needed more carbon atoms to reach saturation, and thus the carburizing time required increased. In addition, the diffusion coefficient of carbon in steel became larger, resulting in the number of surface carbon atoms entering inside increasing. Under the joint effect of these two factors, both the overall average carbon flux and segmented average carbon flux values of steel samples increased with carburizing temperature. The effective carbon flux increased by more than 30% on increasing the temperature from 920 to 980 ◦C. Furthermore, the time required for the carbon concentration on the sample surface to reach saturation also increased with the carburization temperature, which is consistent with the results of this experiment.
